Players from outside the selected 31-man squad took part in a practice session on Thursday ahead of their Pool A game against England this weekend.

"World Rugby is satisfied no deliberate breach was intended, and in addition to the formal warning to the Welsh team, all participating teams have been informed that additional players from the extended national team squad, including potential injury replacements, may not participate in any training sessions," World Rugby said in a statement.

Wales coach Warren Gatland was mystified by the warning, insisting they haven't broken any rules, and he's not sure what's meant by breaking the spirit of the rules.

Wales have had a number of injury problems, and they've contacted tournament organisers about the possibility of bringing in a front-row replacement after props Paul James and Samson Lee sustained injuries in their opening win over Uruguay.

Assistant coach Rob Howley says they've already contacted tournament officials and there is special dispensation for front rows which allows teams to call up replacements as late as 24 hours before a game.

Lee has now been named among the Welsh replacements for the England game.